What’s new in grain storage? – ProFume® fumigations, fumigating large silos and grain protectant update

Take home message

• ProFume® (sulfuryl fluoride gas) applied by licenced fumigators to control storage pests in cereal grains, is valuable when rotated with phosphine fumigations to manage insect pest resistance.
• ProFume trials show that longer fumigation times of 7-10 days are required to control the full life cycle of storage pest insects when grain temperatures are below 25°C.
• In larger silos (150 – 2000 t) recirculating fumigation gases within the sealed silo using a small fan, helps ensures rapid, uniform distribution of phosphine, or ProFume® (sulfuryl fluoride gas).
• Without recirculation during fumigation, it can take 2-5 days before the fumigant gas reaches all areas in a large silo, resulting in significant volumes of grain and insect pests being exposed to low amounts of gas.
• Seek good advice prior to applying any grain protectant treatment. Set up grain protectant spray application equipment to achieve good coverage and the correct dose rate.

Key storage management tools

Fumigations and strategic use of grain protectant insecticides are only two of the five key tools used to maintain grain quality and achieve reliable pest control results. Combined, they form the foundation of successful grain storage. Successful grain storage is crucial to a producer building a reputation as a reliable supplier of quality grain. Key aspects of successful grain storage are:

1. Aeration: correctly designed and managed, it provides cool grain temperatures and uniform grain moisture conditions. Aeration reduces problems with moulds and insect pests in storage, plus maintains grain quality attributes such as germination, pulse seed colour, oil quality and flour quality.
2. Hygiene: a good standard of storage facility hygiene is crucial in keeping background pest numbers to a minimum and reducing the risk of grain contamination.
3. Monitoring: monthly checking of grain in storage for insect pests (sieving / trapping) as well as checking grain quality and temperature. Keep a monthly storage record to record these details, including any grain treatments applied.
4. Fumigation: in Australia we now only have gases (fumigation) to deal with insect pest infestations in stored grain. To achieve effective fumigations the storage/silo must be sealable – gas-tight (AS2628) to hold the gas concentration for the required time.
5. Grain protectants: used on specific parcels of grain like planting seed held on farm, or bulk grain where potential grain buyers have agreed to its use, grain protectant sprays provide another line of defence against storage pests.

ProFume use in Australia

ProFume (sulfuryl fluoride gas) has only been available for use in Australia for a relatively short time (10 years). Phosphine fumigation products have been used to control grain pests for well over 50 years.

Initially registered and sold in Australia by Dow AgroSciences™, ProFume is now manufactured and supplied by Douglas Products™ based in America. A-Gas Rural® based in South Australia has the importing and distribution rights for ProFume. They also provide specialist product and safety training to licenced fumigators, allowing them to purchase and undertake ProFume fumigations.

One of the main drivers for use of ProFume is the continued development of phosphine resistance in storage pests over the past 30 plus years in Australia. Thankfully, for most grain producers, the current levels of phosphine resistance for most storage pest species still allows for complete control when fumigating in correctly sealed, gas-tight silos and when used as specified on the product label.

About 10 years ago one of the flat grain beetle species, known as the rusty grain beetle (Cryptolestes ferrugineus) developed a very high level of phosphine resistance at a number of eastern Australian sites. To control infestations of strongly resistant rusty grain beetles, most bulk handlers and a number of farm storage sites have been able to utilise ProFume.

Figure 1. Flat grain beetles, Cryptolestes spp.

When should I consider using ProFume?

• Phosphine fumigation failure. If live flat grain beetles (Cryptolestes spp) are foundin grain after a well mananaged fumigation, consider using a ProFume fumigation.
• Fumigation resistance management. As for most Ag chemical use, aim for a rotation of products and active ingrediants to combat pests. If phosphine fumigations are often used for pest control at your grain storage facilities, consider a plan to use ProFume® every third year in rotation with phosphine.

Figure 2. ProFume

Key features of ProFume

• ProFume active ingredient is 998 g/kg sulfuryl fluoride. Each gas cylinder holds 56.7 kg.
• Only licenced fumigators with ProFume training can purchase and apply ProFume.
• Registered for use on cereal grains, NOT pulses or oilseeds.
• Requires a gas-tight (sealable silo) storage to hold the specified gas concentrations for the required time.
• Bulk grain treatment costs range from approx. $2-4/t excluding GST, depending on tonnage and travel.
• The ‘eggs’ of storage pests are usually the hardest life cycle stage to kill with ProFume. Longer fumigation times are required.
• Cooler grain temperatures below 25°C, typical for aerated grain, also require longer fumigation times.
• Fumigation time and grain temperature have the largest impact on successful pest control results with ProFume (see Figure 3).

Figure 3. ProFume gas concentrations and time required at 25 and 30°C for complete control of the rust-red flour beetle (TC), rusty grain beetle (CF), lesser grain borer (RD) and rice weevil (SO)
(Red dotted line is the 1500 CT limit for grain application)

Achieving reliable results and practical steps for ProFume fumigation

• Use only in gas-tight silos and storages. Pressure test silo, repair any leakage points.
• Avoid last minute, rushed ‘short’ fumigations. Longer fumigation times in a well-sealed storage provide effective pest control to all life cycle stages including the egg stage.
• Grain at temperatures below 25°C, fumigation times of 7-10 days would be recommended for effective pest control (see Figure 3).
• ProFume (sulfuryl fluoride) is a ‘heavy’ gas. Its’ vapour density = 3.7 (air = 1). ProFume gas is typically applied into the top headspace of a silo. Discuss placement of a sealable fitting at the top of your silo with your licenced fumigator in preparation for fumigation.
• Recent field trials suggest that due to ProFume vapour density, there may be significant benefits to using recirculation during fumigation. This can reduce the tendency for this ‘heavy gas’ to fall and sit at much higher concentrations at the bottom of the silo or storage, leaving insects at the top exposed to much lower concentrations during fumigation.
• Follow all safety requirements as outlined by the licenced fumigator, including leaving fumigation warning signs and safety tape barriers in place. Aeration fans if fitted on storages simplify the venting requirements following fumigation. After venting and prior to grain movement, fumigators will test gas safety levels and ‘clear’ the grain. Keep copies of fumigation documentation.

Fumigation of larger silos (150 - 2000 t or greater)

The first step – ensure “gas-tight storage”

To control live insect pests in grain the only registered products in Australia are now a range of gases. Most often various phosphine fumigation products, and sometimes sulfuryl fluoride gas (ProFume). The controlled atmosphere method is also effective, using either carbon dioxide or nitrogen gas, but is mostly used for pest control in organic grains.

For any fumigation to be effective at controlling storage pests, the insects need to be exposed to a given gas concentration “C“, for a specified length of time “T”. If this “C x T” exposure requirement is not met during the fumigation, it is common to see survival of various insect life cycle stages. With these fumigation failures, live insect pests quickly appear in the grain within days or weeks.

This is why it is critical for Australian grain producers who store grain for more than a month, to have at least two or more sealable, gas-tight storages that meet the Australian silo sealing standard (AS2628).

A storage that is not gas-tight does not allow the fumigation “C x T” exposure level to be reached in all parts of a silo, large or small. Achieving reliable pest control results is not possible with gas leakage and air dilution. As well as not killing the pests, selection and development of resistant insect populations is the additional negative outcome of poor fumigation attempts.

To achieve effective fumigations, silos must be pressure tested to check they are sealed – gas-tight. This ensures they hold high gas concentrations for the required time to kill pests.

Checking a large silo is ready for fumigation – useful equipment for pressure testing

• Portable leaf blower, or small aeration fan, used to add air to silo for pressure tests. High volume, low pressure air is required. Standard air compressors are generally not suited to this task.
• 50 mm poly fitting, including a 50 mm shut-off value, fitted into external section of silo aeration ducting. Using this port to blow air into silo.
• Plastic tube manometer, or better, a digital manometer (e.g. Extech HD 755 Differential pressure manometer 0 – 0.5 psi). Aiming to measure within the range of 0-4 inches water gauge (w.g.) (0-1000 Pa).
• Spray bottle containing water & detergent, to check for leaks. Often you can hear or feel air leaks from large silos during the pressure test.

Pressure test – methods

New silos should be pressure tested by the silo supplier or manufacturer when completed on site. They should pass the Australian standard test (AS2628) to show they are sealable to a standard to allow for effective fumigations.

Sealable silos should then be pressure tested at least once a year to check for suitability for fumigations. Ideally pressure test when a silo is full of grain. This places grain pressure on all silo surfaces and outlets, which is the condition the silo is in when you are fumigating.

Pressure tests should not be conducted in the heat of the day, when the sun is heating the silo’s external steel surfaces and warming / expanding the air inside the silo. The pressure test results under these conditions are meaningless. Ideally test in the early morning before the silo is being warmed. A windy day is also difficult, as silo surfaces are pushed around. Hook up the digital manometer, or plastic tube manometer to the silo when the silo is fully sealed. This will quickly show if pressures inside the silo are stable. If stable, a reliable pressure test can be conducted to test the silo seal quality and for any leakage points.

For small silos the pressure tests can be carried out by using a short burst (5 – 15 seconds) from the small aeration fan fitted to the silo. For larger silos a portable leaf blower to push air into the silo via a fitted 50 mm port can be used to initially pressurise the silo for a test. The pressure decay (250-125 Pa) can be checked using one of three options - the silo’s oil bath relief valves, a length of 20 mm clear plastic tube in a “U” shape with water in it (manometer), or a digital manometer connected to the silo. See GRDC Fact Sheet: “Pressure testing sealable silos”.

Common leakage points for large sealable silos

• Silo roof vents not sealing – maintenance or design problems.
• Silo grain fill point at top of silo not sealing – damaged rubber seals on lid, or sealing plate.
• Grain outload auger at base of silo – leaking seal plate.
• Bottom silo access manhole into silo - damaged seals, or poor design.
• Sealing plate covers for the aeration fan’s intake, often poor design.
• External aeration fan ducting, or the aeration fan itself not well sealed.
• For all cone based silos, weight of grain in the silo can break the seal of the bottom outlet – poor design.

Fumigation recirculation – why is it important for fumigation of larger silos > 150 t

During fumigation, phosphine gas is typically liberated over 5-6 days from tablets or blankets that have been placed in the silo. This gas however only moves slowly, taking about 24 hours to travel 6m through grain.

If you are fumigating a medium to large silo (150 – 2000 t) the gas may take 2-5 days to reach all parts of the silo. In large silo fumigations this may result in some grain, at the furthest distance from tablets, only getting 6 days of phosphine gas instead of the required 10 days or longer exposure period. Six days is not enough time to kill all the life cycle stages of the pests.

One example of a typical phosphine fumigation required to kill all pests, is a minimum of 200 ppm phosphine gas concentration for at least 10 days. See horizontal blue line in Figure 4 below.

Figure 4. Phosphine gas concentrations at 7 points in a silo during fumigation of 1420 t of wheat. Phosphine blankets were placed in the silo headspace with no recirculation. It took as long as 5 days for all grain at the silo base to reached at least 200 ppm gas concentration.

Figure 5. Phosphine gas concentations in a silo (1420 t wheat) where a small fan was used to draw gas from blankets in the silo headspace and pump it into the silo base via aeration ducts for the first 5 days of fumigation. Gas concentration in all areas of the silo reached over 800 ppm within the first 24 hrs.

Figure 6. A small fan (F370 – 0.37 kW) used during the first 5 days of fumigation to recirculate phosphine to give rapid uniform gas distribution in 1423 t wheat. See Figure 5.

Options for fumigation recirculation

• For all fumigation recirculation systems, the sealable silo needs to be gas – tight so there is no gas leakage during the fumigation. In Figure 4, “Base wall north” shows the impact of a leak at the silo manhole, causing large daily fluctuations in gas concentrations.
• Phosphine blankets or tablets can be placed in the ‘silo headspace’ along with a small fan connected to the headspace via 90 mm pipe plumbing coming down the silo wall from the roof. Phosphine gas is drawn from the headspace and pumped into the base of the silo via both aeration ducts (see Figure 5).
• For ground level application of tablets or blankets, a sealable ‘phosphine box’ can be plumbed into this system, either a moveable box, or mounted permanently on each silo.
• Using a fan to force the phosphine gas movement around in silos during fumigation is generally recommended, rather than relying on a passive ‘thermosiphon’ approach. For medium and large silo fumigations, 150 t or greater, or silos storing smaller grain sizes (e.g. millets, canola, lentils etc.) that reduces air movement, fan force recirculation rather than thermosiphon is advised. Fan forced recirculation may also assist where the grain type (e.g. oilseeds) typically absorbs higher amounts of phosphine during fumigation.

Equipment for fumigation recirculation

• Sealable silo - gas tight, that passes a pressure test.
• Plumbing pipes (90 – 100 mm) from silo roof to ground level. Use quality pipe, fittings and seals that will ensure many years of safe, gas- tight fumigations.
• Small fan (e.g. Downfield F370 - 0.37 kW) to recirculate air. In most case this fan size will be suitable for both small & large silos. In trials (Fig. 4 & 5) this fan size provided a complete silo air change every 12 hours for the full silo holding 1420 t of wheat.
• Fittings for fan intake and outlet. Flexible hoses (50 – 100mm) couplings and gate valves.

Fumigation recirculation - operations

• Pressure test the silo to check for leaks.
• Follow all label directions and place tablets / blankets in the ‘headspace’ or ‘phosphine box’.
• Run small recirculation fan for first 5 days of fumigation. Leave silo sealed for remaining days of fumigation exposure period as label requires (e.g. 7, 10, 20 days).

Notes

There are benefits to using the silo ‘headspace’ to locate the blankets or tablets. The large surface area of grain in the headspace provides safe, large easy access for liberated gas to penetrate and diffuse into the grain.

Licenced fumigators commonly choose to use ‘gas’ formulations of phosphine to undertake fumigations in large silos and other storage types, rather than using the solid phosphine formulations of blankets or tablets. An example is Cytec’s ECO₂FUME® containing 20g/kg phosphine in carbon dioxide handled in 31 kg liquefied gas cylinders. While applying the full dose of phosphine gas on day one into a storage has benefits, in many cases the use of a recirculation systems is valuable to provide rapid, uniform gas concentration distribution throughout the storage.

Warning

Always seek advice from a suitably qualified professional before fitting fumigation recirculation systems to silos / storages. Some systems that are currently sold are not recommended because of unsafe design features. Phosphine is not only a toxic gas, but can be flammable and explosive if restricted in a small area, or used in a manner that causes gas concentrations to rise quickly to high levels. Follow label directions and seek advice.

Grain protectant sprays update

Warning

Grain protectant notes below do not apply to the grains industry in Western Australia where their use is restricted. In all cases, product labels are to be used to determine correct use patterns.

When to use grain protectants

• Grain protectant sprays are not to be used to disinfest grain. When live insects are detected, fumigation in a sealed silo is required for effective control.
• Typically, protectant sprays are applied to clean cereal grain at harvest time as grain is augered into storages, providing storage pest protection for 3-9 months. Protectants are effective at controlling insects as they invade or emerge from eggs within grain during storage.
• With many domestic and export markets seeking grain supplies which are “pesticide residue free” (PRF), always talk to potential grain buyers / traders prior to applying grain protectant sprays.
• With the exception of some chlorpyriofs-methyl products in lupins in Victoria only, NOprotectant sprays can be applied to pulses and oilseeds.

Common ‘on-farm’ uses for grain protectants

• Planting seed held on-farm – wheat, barley, oats.
• Grain held for an extended time in non-sealable storages (not suited for fumigation) and grain buyer has agreed to grain protectant use that is in line with directions for use on the registered product label.
• Grain held on-farm as feed for livestock with agreement from livestock agent or buyer and is in line with directions for use on the registered product label.

Grain protectant choices

Examples of two products, which include a partner product, to control the main storage pest species:

1. Conserve Plus™ Grain Protector – a.i. 100g/L spinosad, 100g/L s-methoprene. Used in combination with a compatible organophosphate (OP) product such as chlorpyrifos-methyl (Reldan^TM), or fenitrothion

   For label and details on product use, see: http://www.conserveonfarm.com.au/en

   Recent key recommendations:

   • Always add the OP partner to Conserve Plus so rice weevil (Sitophilus oryzae) is controlled.
   • Spray equipment calibration and application care are critical to achieve correct dose and uniform coverage on grain.
   • If treated grain is exposed to light, for example a semi open grain shed, cover the grain surface with a tarp or 80 - 90% shade cloth. Sunlight breaks down Conserve Plus over time
   • Take care to read notes on the web site (above) and seek advice when purchasing Conserve Plus.
2. K-Obiol® EC Combi, synergised grain protectant – a.i. 50g/L deltamethrin, 400g/L piperonyl butoxide. Used in combination with an organophosate (OP) partner e.g. chlorpyrifos-methyl or fenitrothion.

For label and details on product use, see: https://www.environmentalscience.bayer.com.au/K-Obiol/About%20K-Obiol

Key recommendations

• To control rice, maize and granary weevils (Sitophilus spp.) add a recommended partner (e.g. OP) to the tank mix.
• To ensure effective pest control and that MRL’s are not exceeded, calibrate spray equipment and aim for even treatment / coverage on grain.
• Grower users are required to complete a brief (approx. 60 minutes) online training course to be an ‘approved user’ prior to purchase of K-Obiol® EC Combi. See above web site.

Insect resistant management

If possible, aim to rotate chemical active ingredients for storage pest control at your storage facility. An example, two years use of Conserve Plus™ product combination, followed by one or two years of K-Obiol® EC Combi.

Please read and follow all label recommendations and ensure that the product is registered for use in your state prior to application of any product.

Application for grain protectants

Grain protectant application requires care to achieve the correct dose and uniform grain coverage. This leads to effective pest control results and ensures MRL’s are not exceeded. See Figure 7 below.

• Auger’s grain transfer rate. Ensure you have good understanding of the grain flow rate, tonnes per hour, for the particular height the auger will be operating at.
• Calibrate your spray application unit with water and check appropriate nozzles and spray pressure are used to achieve the required application of 1 litre of spray mixture per tonne of grain.

Figure 7. Spray application equipment designed for good coverage by applying treatment at two points in the auger

Further information

GRDC booklet – Fumigating with Phosphine other fumigants and controlled atmospheres 

GRDC Fact sheet – Pressure testing sealable silos

A-Gas rural – ProFume®

GRDC video – Fumigation recirculation 

Dow™ AgroSciences - Conserve Plus™ Grain Protector

BAYER CropScience - K-Obiol® EC Combi

Acknowledgements

The authors acknowledge the Plant Biosecurity Cooperative Research Centre, of which GRDC is a partner, specifically projects PBCRC3036 and PBCRC3150 under which the fumigant research was conducted. The authors would also like to thank DAF’s Postharvest research team members, GRDC’s national grain storage extension team, along with valued support from growers and other industry collaborators.

Contact details

Philip Burrill
Department of Agriculture & Fisheries, AgriScience Qld.
Hermitage research facility, 604 Yangan road, Warwick Qld. 4370
Mb: 0427 696 500
Email: philip.burrill@daf.qld.gov.au

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